NOS2 and triple-A syndrome: However, excessive ROS generation due to dysfunction of either enzymic and non-enzymic mechanisms [such as enhanced activation of xanthine oxidase (XO), lipoxygenase (LOX) and NADPH oxidase (NOX), overexpression of iNOS and impaired mitochondrial respiratory electron transport chain (ETC)] is closely associated with a number of vascular diseases, including AAA (94, 95).